Delta Delta Delta was founded at Boston University on November 27, 1888, which fell on the day before Thanksgiving that year. Founders' Day is celebrated on the Wednesday before Thanksgiving. I think it's the only NPC group to have a flexible Founders' Day.
There's a little more info at
http://wp.me/p20I1i-tC if you're interested. I'm traveling and cutting and pasting on this little screen is impossible.